1. Field of the Invention
The present invention relates to an image processing apparatus which carries out predetermined image processing on input image data and outputs them.
2. Description of the Related Art
In an original to be used in a printer, PPC (plane paper copier), or the like, a half tone image having density, such as a photograph, and a character image comprising drawings and characters sometimes coexist.
Image processing which is appropriate for either a half tone image or a character image needs to be carried out thereon respectively. Therefore, in the case of such an original, it is necessary to judge the areas of photographs and characters and to switch to image processing appropriate for each.
In Japanese Unexamined Patent Publn. No. 8(1996)-51538, the present applicant has proposed an image processing apparatus which judges a binary image area and a half tone image area by a grade and converts density by selecting a density conversion curve appropriate for each of the images above.
FIG. 2 is a block diagram showing a configuration of the image processing apparatus disclosed therein.
Image data having been read by an image sensor 50 are input to binary conversion means 51 for character processing and binary conversion means 52 for photograph processing.
The binary conversion means 51 for character processing outputs the result of simple binary conversion on the input image data by using a single threshold value.
A plurality of density conversion curves (A, B, C, and D) shown in FIG. 3 are stored in advance in gamma correction means 55. Prior to binary conversion by the binary conversion means 52 for photograph processing, judging means 56 selects one of the density conversion curves for each target pixel based on the input image data. Some of the density conversion curves are appropriate for processing a character image and some are not. As a result, the binary conversion means 52 for photograph processing appropriately carries out binary conversion on each target pixel in the image data comprising both characters and photographs by use of the appropriately selected density conversion curves.
Among the image data having been binary converted by the both means, image data selected in advance by an operator are output selectively by image processing selecting means 53.
The above apparatus is configured so that binary conversion of a half tone image is carried out by the binary conversion means 52 for photograph processing which uses a predetermined error diffusion method.
As in the above case, when there is only one image processing method for a half tone image area, the choice made by an operator is very limited and it is not guaranteed that desired print is obtained with certainty.
As a method of image processing for a half tone image, 2 kinds of methods, namely the error diffusion method and a dither method, have been generally used.
The dither method excels in tone expression of a half tone image, while it has a problem in reproducing an edge or a character when the edge or the character is located in a half tone image area on an original. Furthermore, since it fundamentally has periodicity, it has a drawback that it tends to create moirxc3xa9 of a mesh dot original.
On the contrary, the error diffusion method is good at reproducing an edge or a character when the edge or the character is located in a half tone image area in an original, and it has a characteristic that it is not prone to created moirxc3xa9 when a mesh dot original is processed by this method. However, in tone expression of a half tone image, graininess becomes worse and a problem in smoothness occurs.
A variety of image processing methods other than the above have been proposed as half tone image processing, and each of them has advantages and disadvantages.
As described above, a configuration wherein an optimal image processing whose advantages are utilized in response to the content of a half tone image in an original is carried out by switching has been desired.
The present invention has been created in order to solve the problems described above. The object of the present invention is to provide an image processing apparatus which enables an operator to select image processing on a half tone image area among a plurality kinds of image processing, and carries out image processing suitable for the content of an image.
To solve the problems described above, an image processing apparatus of the present invention which comprises judging means which judges the degree of likelihood of being a half tone image pixel or a character image pixel of a target pixel in an original and outputs a judged degree in stepwise values, gamma correction means which has a plurality of density conversion curves showing relations between input density of image data and output density thereof and converts the image data by using the density conversion curve corresponding to the judgment result by the judging means, and binary conversion means for photographic processing which carries out binary conversion on a half tone image by using a predetermined method, is characterized in that it further comprises:
an image processor installed in the binary conversion means for photographic processing, which carries out binary conversion processing on the half tone image by using at least two different kinds of image processing methods and outputs binary-converted image data; and
a selector which selects a binary-converted image data output by any one of said at least two kinds of image processing methods.
The binary conversion means for photographic processing may comprise
error diffusion processing means which carries out an error diffusion method wherein density difference created when input image data as a target pixel are binary converted by using a threshold value is dealt with as an error and the target pixel is diffused to its surrounding pixels scanned afterwards; and
dither processing means which carries out dither processing wherein a half tone image is expressed by changing density of black pixels in a minute area.
An operator sets in advance which half tone image processing is used for binary conversion of image data of an original.
Whether the input image data are likely to be of a character or a half tone image are judged by the judging means on each pixel in the input image data. Based on the judgment result, an optimal density conversion curve is selected among the plurality of density conversion curves, and the density of the target pixel is converted by the density conversion curve having been selected.
The density-converted image data are processed according to the half tone image processing which has been set.